For bb, you need to overwrite the pluggable functions responsible for logging dudes in. They’re all in bb-includes/pluggable.php, and you overwrite them as you would a wordpress pluggable: https://codex.wordpress.org/Pluggable_Functions

Assuming you are actually talking about bbPress, the best solution I can think of is to make your cookies expire a long time in the future. Then you just need to login once for each user as long as the cookie is there. Anonymous posting doesn’t sound like a solution as you probably still want to know who owns which messages.

To set the cookies to expire after ten years, save this to a file called “_loginExpiry.php” in my-plugins: